In a known electric power steering apparatus for a vehicle, rotational output of an electric motor for providing an auxiliary steering torque is transmitted, while reduced, to an output shaft of the steering apparatus by a gear apparatus to assist the steering force applied to the steering wheel in the wheel steering operation. In such an electric power steering apparatus, the power is transmitted to the output shaft while the rotation of the electric motor is reduced by a power transmission mechanism provided in the housing.
The mainstream electric motor used in such power steering apparatuses used to be a motor having a brush. However, in recent years, it has been shifting to a brushless motor in order to improve reliability and to further enhance efficiency.
An electric power steering apparatus in which a brushless motor is employed is shown in FIG. 16. The output shaft 1 is coupled with, via a torsion bar (not shown) etc., an input shaft (not shown) that is, in turn, coupled with the steering wheel (not shown), and a worm wheel 2 of a worm gear reduction mechanism is fixedly attached on the output shaft 1.
A worm 5 rotatably supported on a gear housing 3 by means of a bearing 4 etc. is in engagement with the worm wheel 2.
With the gear housing 3, a motor cover 7 for the brushless motor is coupled via a flange serving as a partition plate. The rotor 10 of the brushless motor is rotatably supported on the flange 6 and the motor cover 7 via bearings 8 and 9 respectively. The rotor 10 and the worm 5 are linked by means of a spline fitting portion 11 so that they are movable in the axial direction but non-rotatable relative to each other.
In the radially outer periphery of the rotor 10, there is provided a cylindrical permanent magnet 12 for rotational driving in such a way as to be opposed to a laminated core 16. In addition, a ring-shaped permanent magnet for sensing 13 is also provided.
Radially inside the motor cover 7, a cylindrical stator 14 is provided. The stator 14 is formed by resin molding integrally including the laminated core 16 on which a coil 15 is wound, a hall IC 17 for detecting magnetism of the permanent magnet for sensing 13, a board 18 on which the hall IC 17 is mounted and a terminal (busbar) 19 for providing connection for the coil 15.
To the terminal (busbar) 19, an electric cable 20 for a terminal connected with the coil 15 and for a terminal for outputting a signal of the hall IC 17 is joined by soldering or other means.
In recent years, with an increase in the types of vehicles in which electric power steering apparatuses are to be employed, the output power demanded for the power steering apparatus has been increasing. Thus, an increase in the output power of the motor is demanded, and on the other hand, downsizing thereof is also demanded for the purpose of space-saving.
If the hall IC 17 is disposed at a position too closed to the coil 15, a malfunction of the hall IC can occur due to the magnetism generated by the coil 15. Accordingly, it is necessary to dispose the hall IC with a certain distance (a certain space) from the coil 15. Therefore, it is difficult to reduce the length of the motor.
Furthermore, it is necessary to provide a bracket and flange for supporting the bearings 8, 9 on the outer periphery or the end faces of the stator 14, and therefore it is difficult to reduce the axial length of the rotor 10 with the bearings 8, 9.
International publication WO99/65758 discloses a brushless motor in which a common bearing is used for the rotor and the worm in order to reduce the length of the motor.
However, in that arrangement, the axial position of the rotor will be displaced with variations in the load of the worm, which results in problems such as generation of vibration or changes in characteristics. In addition, the assembly operation is required to be carried out under strict control in order to prevent attachment of iron dust etc. to the rotor provided with a permanent magnet for rotational driving, and it is necessary to assemble the rotor and the stator separately. Thus, there arise problems such as a deterioration in ease of the assembly operation and an increase in the process steps in the assembly operation. Furthermore, the motor cannot be subjected to inspection until assembly of the worm gear mechanism and the brushless motor has been finished, that is, inspection in terms of its performance etc. cannot be performed for the motor in a stand alone state.
In the electric power steering apparatus shown in FIG. 16, in the boundary space between the worm gear mechanism and the brushless motor, there is provided the aforementioned flange 6 (partition plate) and the bearing 8 mounted on the radially inner circumference of the flange (partition plate).
Therefore, if grease or foreign matters such as iron dust in the worm gear mechanism are about to enter the brushless motor, entry of the foreign matters and grease can be prevented by the seal achieved by the aforementioned flange 6 (partition plate) and the bearing 8.
On the other hand, if downsizing of the brushless motor is intended, a structure in which the aforementioned bearing 8 is eliminated or a structure in which the mount position of the bearing 8 is changed may be adopted.
However, if the bearing 8 is eliminated or the position thereof is changed, the boundary portion between the worm gear mechanism and the brushless motor becomes open, and the worm gear mechanism and the brushless motor are in communication with each other. As a result, there is a risk that grease or foreign matters such as iron dust in the worm gear mechanism will enter the brushless motor.
Furthermore, brushless motors formed by resin molding have been desired from the standpoint of downsizing of the electric power steering apparatus, motor noise reduction or facilitating heat radiation of the motor.
However, there are no motors formed by molding employed in electric power steering apparatuses presently, though driving motors used in ventilators of consumer electrical appliances are generally motors formed by molding.
A motor formed by molding used for driving a ventilator will be described with reference to FIGS. 17A and 17B (see page 2 and FIGS. 1, 2, 9 and 10 of Japanese Patent Application Laid-Open No. 10-271720, for example). FIG. 17A shows a motor 200 for a ventilator as viewed from obliquely above, and FIG. 17B is a cross sectional view of the motor 200. In FIGS. 17A and 17B, a stator 201 is molded with synthetic resin 204 in such a way as to envelop a stator iron core 202 and winding 203. On the other hand, to a rotor 208 integral with a shaft 211, a permanent magnet 209 is attached. They are housed in the stator 201, and the shaft 211 is supported by a bearing 206 received in a bearing housing 205 and a bearing 214 received in a bearing bracket 213. In this bearing structure, although it is difficult to achieve a high coaxiality of the stator 201 and the rotor 208, its application as a motor for a ventilator of a consumer electric appliance does not require precision more than necessary in the coaxiality.
If a brushless motor for an electric power steering apparatus having a structure similar to the above-described motor for a ventilator is formed by molding, the problem described below will arise. In the case of the motor for an electric power steering apparatus, when the steering wheel is turned in one direction then the other upon driving the vehicle to a parking space or when the steering wheel is turned quickly at a critical moment, the motor is required to rotate in forward and backward directions with a large torque. Therefore, extremely high precision in the coaxiality is demanded in order to suppress noises and torque ripples.
In the bearing support structure for the rotor provided with a bearing chamber formed by resin integral molding, it is generally difficult to enhance precision of the bearing chamber due to variations in conditions during the molding, differences in the contraction percentage depending on the resin molding directions, weld or other reasons. In the case that the precision of the bearing chamber is low or its coaxiality with the inner circumference of the stator is low, the coaxiality of the cylindrical stator and the cylindrical rotor inserted and adapted to rotate therein is deteriorated. This invites, as consequences, an increase in the cogging torque, an increase in the friction torque and an increase in the motor noises. The increase in the cogging torque will be transmitted to the driver's hands directly to give him/her unpleasant feeling, and the increase in the friction torque leads not only to a motor power loss but also to deterioration of response characteristics to the motor control signal.